Various communication methods which allow communication to be carried out without link-block of transmissions in the same channel are employed in a network in which the same channel is shared by plural communication terminals. For example, CSMA (Carrier Sense Multiple Access) method, TDMA (Time Division Multiple Access) method, polling method and the like are employed.
In the CSMA method, transmission data is put into a packet of a predetermined size and transmitted. A communication terminal using this method transmits the packet using a carrier having a predetermined frequency. Upon transmission of the packet, the carrier is detected (the term carrier detection is used in this specification) in order to investigate whether or not there is other packet in a channel (transmission path) for use in the transmission.
When a channel for use is found to be empty as a result of detection for the carrier, the packet transmission is carried out. This CSMA method is suitable for LAN (Local Area Network) in which transmission speed is about 1 Mbit/second-10 Mbit/second.
According to the TDMA method, a plurality of so-called time slots are formed by dividing transmission time in terms of time, so that the time slots for transmitting the packet are set up fixedly for each communication terminal at the time of communication start-up. Then, because each communication terminal transmits the packet within a time slot allocated to itself, apparently it looks as if plural communication terminals use the same channel (a single high-speed communication line) at the same time.
According to the polling method, a communication control unit inquires each of plural communication terminals using the same channel of whether or not it makes a communication request. Based on this request, the communication control unit determines a sequence of communication to a communication terminal which makes a communication request and then carries out communication.
By using these communication methods, the same channel (transmission paths) can be used by plural communication terminals without generating any collision of transmission.